This invention relates generally to solid state imager circuits, and, more specifically, to pixel readout and buffer circuits utilizing switched capacitors to achieve an improved signal-to-noise ratio and improved signal gain.
The primary function of a solid state imager is the sensing of photons, and the conversion of their energy to a measurable electrical voltage signal. Typically, the voltage generated by moving charge from a photodiode to a capacitive node in a pixel element is sensed. A reset operation is applied to each pixel element to determine a reference level for each diode in the array of pixel elements. Following the pixel reset operation, the pixel elements are again sequenced to read a sampled pixel signal. The difference between the sampled pixel signal and the reference level is determined for each pixel element in the array.
The primary performance parameter for an active pixel sensor, whether used for charged particle spectrometry, or for imaging, is the signal-to-noise ratio. The noise can be divided into spatial, temporal and detection components. The spatial, or Fixed Pattern Noise (FPN), is generally the largest noise source and of significant applicability in the present invention.
During the conversion of the captured electron charge to voltage (exe2x88x92 to V), the resulting sampled signal level strength is quite small. Consequently, an amplification, or application of gain, to the resulting sampled signal level is required to obtain a voltage swing having an appreciable dynamic range. Accordingly, it is desirable to apply maximum gain at the signal source to avoid amplifying noise from downstream noise injecting sources.